Abstract: A 42CrMoA steel flange output shaft fractured quickly after being put into use. The causes of the fracture of the output shaft were analyzed by means of macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, metallographic examination and so on. The results show that the overburning of the structure due to the high heating temperature in the early stage of forging was the main reason for the fracture of the output shaft. After overburning, the grain boundary bonding force became weak, the material deformation ability was insufficient, and microcracks were formed at the position where the cross-section deformation was large. The stress concentration and mechanical properties did not meet the requirements, resulting in rapid crack propagation. Finally, the flange output shaft brittle fracture occurred.